1. Field of the Invention
The present invention relates to an automatic focussing system and, more particularly, it relates to an automatic focussing system suitable for use in a photographing camera, a video camera or the like in which a magnitude of focus deviation, corresponding to a defocus, produced particularly when an image is produced through an exchangeable lens system, is detected and a focussing lens of a photographic lens system is adjusted according to the detected magnitude of deviation of focussed position to attain the focussed state.
2. Description of the Related Art
An automatic focussing system in which a focus error of a photographic lens system is detected by focus detecting means provided in a camera body and a focussing lens of the photographic lens system is adjusted according to an output signal produced by said focus detecting means to effect focussing operation has been proposed, such as in Japanese Laid-Open Patent Applications No. Sho 55-11275 and No. Sho 59-151116, etc.
In general, a non-linear relation exists between a magnitude of focus deviation of a photographic lens system (a deviation of an image surface at the time of detection from a predetermined image plane) and a magnitude of movement of a focussing lens which is required to attain correct focussing of the photographic lens system. In case of a mono-focal photographic lens system, for example, the magnitude of focus deviation varies, depending upon such factors as an object distance, etc. In case of a variable-power lens system, the magnitude of focus deviation varies, depending upon such factors as an object distance, a position of a focussing lens along its optical axis, a zooming position, etc.
Such a variation of the magnitude of focus deviation is caused because the ratio .DELTA.x'/.DELTA.x (.DELTA.x is a minute magnitude of movement of the focussing lens along its optical axis and .DELTA.x' is a magnitude of movement in axial direction of position of an image surface), that is sensitivity S, varies, depending upon respective elements of the photographic lens system and photographing conditions.
For example, it is assumed that there is a focus deviation .DELTA.d in a mono-focal photographic lens system and focussing of this lens system is effected by moving the lens system as a whole. Then, if the lens system is moved by a distance equal to .DELTA.d, correct focussing cannot be attained and an error in focussing is produced. Such an error in focussing becomes larger as an object to be photographed is located nearer to the lens system.
Next, it is assumed that there is a focus deviation .DELTA.d in a variable-power photographic lens system in which a first group of lenses at the side of an object constitutes a focussing lens and includes a variable-power variable from f=1 to f=4, for example, and a magnitude of movement of the focussing lens at the time of f=1 is .DELTA.x.sub.W. Then, if the power is changed from f=1 to f=4, the magnitude of movement of the focussing lens .DELTA.x.sub.T changes to .DELTA.x.sub.T =.DELTA.x.sub.W /4.sup.2 . Thus, it will be clear that the magnitude of movement of the focussing lens changes, depending upon the zooming position of the variable-power system, namely the selected power of the variable-power system, even if the magnitude of focus deviation is the same.
On the other hand, at the time when the focussing lens group of the photographic lens system is located at the position where it is focussed to an object at infinite distance and at the time when said focussing lens group is located at the position where it is focussed to a nearer object, the magnitude of movement of the focussing lens required to attain the correct focussing is different, even if the magnitude of deviation of focussed position is the same.
Furthermore the required magnitude of movement of the focussing lens changes, depending upon the object distance, even if the magnitude of deviation of focussed position is the same.
On the reasons as mentioned above, the precise focus detection cannot be attained without deciding the required magnitude of movement of the focussing lens while considering several factors of the photographic lens system, even if the magnitude of deviation of focussed position is detected by the focus detecting means.
Heretofore, in case of the mono-focal lens system, the focussing was effected by moving the focussing lens by the magnitude of focus deviation .DELTA.d or the corresponding magnitude. In case of the variable-power system, the focussing was effected by moving the focussing lens by the magnitude .DELTA.x which was determined according to the formula of .DELTA.d/S.sub.Z. In this case, only the sensitivity S.sub.Z at the respective zooming positions was taken into consideration. In such conventional system, there occurred substantial amount of error in movement of the focussing lens and it was difficult to attain precise focussing by only one time of movement of the focussing lens. According to the prior art, therefore, it was necessary to repeat the movement of the focussing lens a few times to progressively attain the precisely focussed position. Thus the time to attain the focussed position is increased, so that it is difficult to effect a photographing operation in a rapid and precise manner.